Apple Patenting Method to Harden Stainless Steel
Reports that Apple is patenting a process to harden stainless steel has prompted speculation
that the company is interested in making cases for future mobile devices out of steel, rather than plastic or glass. The method described in the application involves placing a thin layer of nitride on top of the steel to improve its ability to resist scratches and rust.
The patent, called "Nitriding Stainless Steel for Consumer Electronic Products
," talks about the rust-resistant quality of stainless steel: specifically, the addition of chromium to the alloy which permits a thin layer of chromium oxide to form on the surface, inhibiting further rust. Some manufacturers use a titanium nitride coating to add further protection to the steel, but this can cause a yellowish tint. Apple's patent presents a way to preserve the natural color of the steel by using a different nitride. "In addition to providing a durable, hard surface that is both scratch and impact resistant," the patent reads, "the nitride layer allows for the natural surface color and texture of the underlying stainless steel to remain visible to the user." The layer would be just 15 microns thick but would have a Vickers hardness rating of over 1000. Normal stainless steel has a hardness rating of about 150.
Austenitic stainless steel, which is a form of stainless that includes nickel, is useful for mobile devices because it is non-magnetic and does not block RF, WiFi, Bluetooth, or other wireless technologies utilizing electromagnetism. The band around the iPhone 4 is made of this material.
Apple's interest in metal manufacturing techniques is not limited to this process: the company obtained an exclusive license
to use the intellectual property of Liquidmetal Technologies to create extremely strong, flexible materials with the pliability of plastic so that they can be formed into a wide variety of shapes.